#include <stdio.h>
#include <math.h>
#define S(a,b,c) (a+b+c)/2
#define area(a,b,c) sqrt(S(a,b,c)*(S(a,b,c)-a)*(S(a,b,c)-b)*(S(a,b,c)-c))

int main()
{
	double x,y,z;
	scanf("%lf %lf %lf",&x,&y,&z);
	printf("%.3lf",area(x,y,z));
	return 0;
}